The chief sniper during the Lindt Cafe siege will sue NSW Police, alleging he and fellow officers were prevented from doing their jobs and the lives of cafe manager Tori Johnson and barrister Katrina Dawson could have been saved.

The officer, who can only be identified as Sierra 3-1, alleges in a 30-page affidavit he has suffered psychological trauma due to decisions taken by his superiors during the siege on December 15, 2014, in which 17 people were taken hostage.

The Daily Telegraph reports the civil action, lodged in the NSW District Court on April 12, will be heard next year in a one-day hearing.

It follows another complaint the sniper made to the Law Enforcement Conduct Commissioner that was dismissed. In it, he claimed senior officers perjured themselves at the initial inquest.

Ms Dawson, a 38-year-old mother-of-three, was killed by a police bullet as the siege ended in a hail of gunfire early on December 15, 2014. Mr Johnson, the cafe’s 34-year-old manager, was executed by gunman Man Haron Monis, a move that sparked the police intervention.

Monis was a 50-year-old self-described Muslim cleric who was on bail on serious charges, including being an accessory to murder and sexual assault. He also died.

In 2017, NSW Coroner Michael Barnes found police waited too long to enter the Lindt cafe to rescue the hostages.

In a 600-page report into the handling of the siege, he found police commanders relied on “flawed advice” from negotiators that overestimated their chances of a peaceful resolution.

-with AAP